As mobile devices have been increasingly developed, and the demand of such mobile devices has increased, the demand of secondary batteries has also sharply increased as an energy source for the mobile devices. Among them is a lithium secondary battery having high energy density and voltage, extended cycle life, and low self discharge rate, which has been commercialized and widely used.
Based on the construction of electrodes and an electrolyte, the lithium secondary battery may be classified as a lithium-ion battery, a lithium-ion polymer battery, or a lithium polymer battery. Among them, the lithium-ion polymer battery has been increasingly used because the lithium-ion polymer battery has a low possibility of electrolyte leakage, is lightweight, is manufactured with low costs, and can be easily constructed in various shapes.
An electrode assembly having a cathode/separator/anode structure, which constitutes a secondary battery, may be generally classified as a jelly-roll (winding) type electrode assembly or a stacking type electrode assembly, based on the structure of the electrode assembly. The jelly-roll type electrode assembly is manufactured by coating a metal foil to be used as a current collector with an electrode active material, drying and pressing the coated metal foil, cutting the dried and pressed metal foil into the form of a band having a predetermined width and length, isolating an anode and a cathode from each other using a separator, and helically winding the anode/separator/cathode structure. The jelly-roll type electrode assembly is suitable for cylindrical batteries; however, the jelly-roll type electrode assembly is not suitable for prismatic batteries or pouch-shaped batteries because the electrode active material may be detached, and the spatial utilizability is low. On the other hand, the stacking type electrode assembly is an electrode assembly constructed in a structure in which pluralities of unit cathodes and anodes are sequentially stacked one on another. The stacking type electrode assembly has an advantage in that the stacking type electrode assembly can be constructed in a prismatic structure; however, the stacking type electrode assembly has disadvantages in that a process for manufacturing the stacking type electrode assembly is complicated and troublesome, and, when external impacts are applied to the stacking type electrode assembly, electrodes of the stacking type electrode assembly are pushed with the result that short circuits occur in the stacking type electrode assembly.
In order to solve the above-described problems, there has been developed an overlapping type electrode assembly, i.e., an electrode assembly constructed in a structure in which full cells having a cathode/separator/anode structure of a predetermined unit size or bicells having a cathode (anode)/separator/anode (cathode)/separator/cathode (anode) structure of a predetermined unit size are sequentially stacked one on another, such that the cathodes face the corresponding anodes, while a long continuous separator sheet is disposed between the full cells or the bicells. Examples of such an overlapping type electrode assembly are disclosed in Korean Patent Application Publication No. 2001-82058, No. 2001-82059, and No. 2001-82060, which have been filed in the name of the applicant of the present patent application. The structure of the overlapping type electrode assembly is clearly shown in FIGS. 1 and 2, which are typical views illustrating a process for manufacturing the overlapping type electrode assembly.
Referring to FIGS. 1 and 2, the overlapping type electrode assembly 10 is manufactured through several steps, i.e., a step of cutting cathodes 1, an anode 2, and separators 5 such that the cathodes 1, anode 2, and separators 5 have a predetermined size, a step of sequentially stacking the cut cathodes 1, anode 2, and separators 5 to manufacture a bicells (or full cells), a step of folding a plurality of bicells 6 using a separation film 7, and a step of electrically connecting electrode taps 3 and 4 protruding from one-side ends of the cathodes 1 and the anodes 2. Consequently, the manufacturing process is complicated, and therefore, the manufacturing costs and manufacturing time are increased. In addition, an additional connecting member is essentially used at the step of the connecting the electrode taps 3 and 4, and a troublesome work, such as welding, is required. Consequently, the manufacturing costs are further increased, and a possibility of battery defect is increased.
In this connection, there has been proposed a method of manufacturing an electrode assembly into a zigzag type overlapping structure in vertical section instead of the stacking or overlapping structure.
For example, Japanese Patent Application Publication No. 2001-160393 discloses a method of folding a separator sheet, while cathodes and electrode stack units, such as bicells or full cells, are attached to the separator sheet, in a zigzag fashion to manufacture an electrode assembly. However, this manufacturing method requires a step of manufacturing the electrode stack units and a step of arranging the electrode stack units on the separator sheet, like the overlapping electrode assembly. Consequently, this manufacturing method has fundamental limits.
In addition, Korean Patent Application Publication No. 2000-51741 discloses a method of constructing electrode sheets and a separator into a stacking structure having a cathode/separator/anode arrangement and folding the stacking structure in a zigzag fashion to manufacture an electrode assembly. This manufacturing method has an advantage in that this manufacturing method is very simple as compared to the manufacturing method disclosed in the above-mentioned Japanese patent application publication. However, the stacking structure having the cathode/separator/anode arrangement has a specific thickness with the result that it is very difficult to fold the stacking structure in the zigzag fashion. Furthermore, electrode active material layers may be separated from electrode current collectors when a force is applied to the stacking structure such that the stacking structure is folded.
Therefore, there is a high necessity for a technology to fundamentally solve the above-mentioned problems.